Neisseria meningitidis is serologically classified into serogroups, serotypes and immunotypes based on capsular polysaccharides, outer-membrane proteins and lipooligosaccharides (LOSs), respectively. The LOS is one of the major surface antigens and can be divided into 12 immunotypes. Epidemiological studies have revealed that the L3 immunotype is the most common immunotype among serogroups B and C organisms causing disease, the predominant serogroups of meningococcal disease in the U. S. and Europe. We have been investigating the potential use of detoxified LOS vaccine for the prevention of group B disease, which currently has no licensed vaccines. We selected an OP- mutant LOS instead of the L3 LOS to prepare the LOS vaccine for two reasons: (1) Rabbit antisera to the OP- LOS cross-reacted with the L3 LOS and (2) unlike the L3 LOS, the OP- LOS is devoid of the lacto-N-neotetraose (LNnT) structure, a precursor of ABO major blood group antigens. Thus, there is no risk of inducing antibodies by the vaccine that may cross react with the LNnT-containing glycolipids such as paragloboside on human cells and tissues. In our study of the LOS-based conjugate vaccine, we used deoxycholate-treated outer-membrane protein (DOMP) from OP- strain as a carrier protein for the synthesis of the LOS-derived oligosaccharide (OS)-DOMP conjugate. We found that the tightly protein-bond LOS in unconjugated DOMP alone which contained about 20% LOS was as immunogenic as the OS-DOMP conjugate in rabbits with a dose of 50 ug LOS. The OP- LOS in DOMP behaved as a T-dependent immunogen and elicited a strong IgG antibody response to the LOS in rabbits. Rabbit antisera cross-reacted with most of 12 LOS immunotypes including L3. Although lethal toxicity in D-galactosamine-sensitized mice and pyrogenicity in rabbits of LOS tightly bound to DOMP was attenuated at least 200-fold, the toxicity of the native LOS in DOMP as a vaccine may still be of concern. Therefore, we reduced the 50 ug LOS dose to one-tenth or one-hundredth and examined the LOS immunogenicity of OP- DOMP in rabbits. Rabbit IgG antibody titers decreased less than one half for one-tenth dose and less than one eighth for one-hundredth dose in response to first and second immunization as measured by ELISA. We will continue to examine the immunogenicity of OP- LOS in DOMP in mice and also evaluate antisera in functional assays such as bactericidal assay.